The present invention relates to a distribution device for dewatering pulp.
When producing paper pulp from cellulose-containing fiber material, there is a need to wash and dewater the paper pulp at one or more points in the process.
A previously known and frequently employed device, termed a wash press, for washing and dewatering paper pulp is shown in SE-C-380 300, SE-C-501 710, U.S. Pat. No. 5,488,900 and SE-C-504 011. The devices which are shown in these publications comprise two cylindrical, rotatable screen members which are arranged in a trough which is in the main convergent but which is partially divergent at the intake for washing liquid. Other examples of known devices are shown in U.S. Pat. No. 4,543,161 and U.S. Pat. No. 5,667,642, with the latter constituting a device in which the screen members rotate in the opposite direction to the conventional direction, i.e., when seen from a short side, the right screen member rotates anticlockwise and the left rotates clockwise.
A problem associated with devices of the prior art, which possess two cylindrical screen members which can be rotated towards each other, is that of obtaining an efficient initial distribution of pulp over the whole of the length of the dewatering device. An example of how this problem has been solved in shown in SE-C-500 546, in which a crescent is to distribute the pulp over a line by means of conducting the pulp from a punctate inlet and over the edge of the crescent. The lowest height of the crescent is obtained furthest away from the inlet and consequently a longer supply distance is compensated by a lower bridging height. In these types of solution, practical limits have been set at pulp concentrations of the order of 8%.
Other solutions have instead employed a distribution screw, similar to U.S. Pat. No. 4,559,104, which distribution screw provides a substantially improved initial distribution over the whole of the width of the wash press.
A good initial distribution of the pulp over the whole of the width of the wash press is a very important parameter for succeeding in obtaining a wash press which possesses high operability, i.e., which does not plug up, with the press subsequently having to be stopped in order to remove plugs, and a high dewatering ability. Ways of dealing with the problems involved in assisting uniform distribution of the pulp over the whole of the width of the wash press have exclusively concentrated on solutions corresponding to SE-C-500 546 or U.S. Pat. No. 4,559,104, and it has only been possible to discern any improvements in the transitions between pulp inlet boxes and trough sweeps in the form of shorter radii between the pulp inlet boxes and the trough sweeps, corresponding, for example, to U.S. Pat. No. 5,667,642.
In some wash presses, use has instead been made of a very long, gradual decrease in the gap from the inlet box, see, for example, SE 380 300. While this provides an equalizing effect, this very lengthy dewatering and equalizing zone at the same time results in a reduced total displacement ability. That is to say that the washing liquid which is to penetrate through the pulp web is given a much shorter stretch to act on, and the pulp web which is to be washed becomes thicker. An inferior displacement ability results in higher residual contents of chemicals in the pulp, something which is a negative feature, chiefly from the environmental point of view and from the point of view of recovery.
One object of the present invention is to provide a wash press whose operability is improved and which can be fed with high concentrations of pulp without there being any risk of it plugging up. When use has been made of a pulp inlet box in accordance with the invention, with the inlet box being employed for a wash press having double circular-cylindrical screen members and pulp inlet boxes at the highest point of the screen member, good operability has been obtained with pulp concentrations of the order of 12% in the inlet.
An additional object is for the pulp web to be relatively rapidly reduced in thickness, thereby providing the possibility of improved washing, i.e., improved displacement ability. It is important to be able to use new washing liquid to rapidly displace the chemicals which are present in the pulp, with these chemicals being returned, where appropriate, to the preceding stage in the pulp preparation process.